U.S. Pat. No. 5,119,774, entitled "Direct Acting Hydraulic Valve Lifter" issued on Jun. 9, 1992 by Krieg et al., describes a direct acting hydraulic valve lifter (DAHVL) having various features which individually and/or in combination may provide reduced reciprocating mass with lower oil loss in operation, faster filling of the lifter after draining and more positive discharge of air from the lifter. Vent means from the oil chambers between the hydraulic element assembly and the follower cylinder supporting it provide for the passage of air within the DAHVL.
U.S. Pat. No. 5,129,374, entitled "Hydraulic Tappet", issued on Jul. 14, 1992 by Flavio, provides structure within the tappet which causes the oil to traverse a 360.degree. circular path and through two vertical levels in flowing from the inlet to the inner reservoir within the tappet.
U.S. Pat. No. 4,615,306, entitled "Engine Valve Timing Control System" issued on Oct. 7, 1986 by Wakeman and assigned to a common assignee, describes a system using electrohydraulic valve lifters operatively connected to an ECU to provide real time changes in engine valve timing. Pressure pulses within the system, operate to "home" the lifters to the base circle of the timing cams. The ECU controls a solenoid which controls the passage of fluid to and from the lifters.
U.S. Pat. No. 5,005,540, entitled "Valve Timing Control System for an Internal Combustion Engine", issued on Apr. 9, 1991 by Watanabe, teaches a DAHVL utilizing two pistons and the ratio of their respective areas to multiply the lift of the timing cam to open the engine valve the desired amount. Fluid is supplied and removed by passageways that are both horizontal and inline.